The present invention relates to a radio communication apparatus and method used in a car telephone, a portable telephone or the like.
In a cellular radio communication system for a car telephone, a portable telephone or the like, as a multiple access system for transmission signals in which a plurality of stations communicate concurrently at the same frequency band, a FDMA system, a TDMA system and so on are known. Recently, a CDMA system (Code Division Multiple Access system) has been applied as a system in which higher frequency utilization is obtained and more system users are accommodated than other techniques such as the FDMA system, the TDMA system and so on.
In addition, there are a FDD system and a TDD system as a transmission system to perform a bi-directional communication of transmission data over forward link and reverse link between a mobile station and a base station in a cellular system. The FDD system utilizes different frequency bands in forward and reverse links. The TDD system utilizes the same frequency band and transmits forward link signals and reverse link signals using time slots. Accordingly, a CDMA/TDD mobile station apparatus is a mobile station apparatus using the CDMA system as a multiple access system and the TDD system as a system for discriminating the forward link and reverse link.
The configuration of a conventional CDMA/TDD mobile station apparatus will be explained below using a block diagram in FIG. 1.
As illustrated in FIG. 1, a mobile station comprises antenna 101 via which data is transmitted and received to and from a base station, switch 102 to switch a time band for transmitting and receiving, RF receiving section 103 to convert the received data passed through switch 102 to baseband signals, AD converter 104 to convert the data converted at RF receiving section 103 to digital data, correlator 105 to despread the data converted at AD converter 104 with a spreading code and decoder 106 to decode the data despread at correlator 105.
Further, the mobile station comprises synchronization circuit 107 to generate synchronization signals used in despreading at correlator 105 from an output from AD converter 104 and to generate frame synchronization signals from the decoded data, and timing generation circuit 108 to generate switching timing signals for switch 102 based on the frame synchronization signals output from synchronization circuit 107.
Furthermore, the mobile station comprises coder 111 to code transmission data, spreading multiplexing circuit 112 to spectrum spread the data coded at coder 111 to multiplex each channel, DA converter 113 to convert the data spread at spreading multiplexing circuit 112 to analog data, and RF transmitting section 114 to convert the data converted at DA converter 113 to RF data.
Furthermore, the mobile station comprises control circuit 120 to control the whole apparatus such as transmission power, frame configuration and so on.
The mobile station configured as described above selects receiving or transmitting by switch 102 corresponding to a control signal output from timing generation circuit 108. In the case of selecting receiving, it decodes the data received from a base station using antenna 101, RF receiving section 103, AD converter 104, correlator 105, and decoder 106. On the other hand, in the case of selecting transmitting, it transmits transmission data to a base station using coder 111, spreading multiplexing circuit 112, DA converter 113, RF transmitting section 114 and antenna 101. The switching timing of switch 102 is provided from timing generation circuit 108 based on the frame synchronization signals generated at synchronization circuit 107. The control of each section in the mobile station is carried out by control circuit 120.
In the conventional CDMA/TDD system mobile station described above, when the mobile station moves from a cell of a base station in current communication to another cell of another base station for next communication, control circuit 120 performs a switching of transmission and reception frequencies or a spreading code in order to communicate with a base station for the new communication.
However, there has been a problem that the conventional CDMA/TDD system mobile station described above has no hand-over system that is efficient to switch a base station quickly, therefore it takes a time to acquire synchronization with the base station for new communication and the mobile station has to discontinue communication temporally while it moves from a cell of a base station in current communication to another cell of another base station for next communication.
It is an object of the present invention to provide a CDMA mobile station apparatus capable of switching a base station in a short time while it moves from a cell of a base station in current communication to another cell of another base station for next communication and preventing transmission property from being deteriorated even in the case where the mobile station is present around a boundary between the cells of two base stations.
The object is achieved according to the following steps: First, a mobile station detects reception power of data received from surrounding base stations, and when a level of the received power from a base station that is not the main base station in current communication becomes higher than a predetermined level, determines this base station as a sub base station, then transmits and receives data to and from the sub base station concurrently along with the main base station. After that, when the received power from the sub base station becomes higher than that from the main base station, the mobile station switches the main base station and the sub base station so as to transmit data to a new main base station.